Heat-sensitive recording materials, which utilize a color-forming reaction between a colorless or pale-colored leuco dye and an organic or inorganic developer in such a manner that the two chromogenic materials are brought into contact with each other by heating to produce a recorded image, are well known. Such heat-sensitive recording materials are relatively inexpensive, and the recording devices are compact and easy to maintain. Therefore, heat-sensitive recording materials have been widely used not only as recording media for facsimile machines, various printers, etc., but also in a variety of fields.
On the other hand, various methods have been used to apply a coating composition onto a base material such as paper, films, etc. Examples of the methods include air knife coating, blade coating, rod coating, roll coating, and bar coating. However, the heat-sensitive recording materials produced by these methods have defects including poor coating quality, interfusion of the coating composition in the upper layer into the under layer, pinholes in an upper layer due to cissing upon application onto the upper layer, and unstable quality due to long-time continuous application. Moreover, these methods also have problems such as limitation of high-speed coating, a decrease in productivity due to frequent applications, and the like.
In contrast to the above coating methods, curtain coating methods (see Patent Document 1, etc.) form a free-falling curtain of a coating composition, in which the composition is applied to a support by impinging it onto the support. Such curtain coating methods are known for their excellent application duality and suitability for high-speed application. Moreover, the curtain coating method may be performed by forming a coating composition film consisting of multilayered coating composition film. This is greatly conducive to an increase in productivity in multilayer coating. According to the curtain coating methods, a coating liquid is formed, into a curtain of fluid while falling from a slit-like coater lip onto a base material to be coated. Therefore, the curtain coating requires precise adjustment in static surface tension, dynamic surface tension, viscosity, etc., so as to improve the film-forming ability of the coating composition or its wettability to the base material, thereby preventing uneven or defective application (see Patent Document 2). In these views, evenness and stability of the application itself are very important, as they greatly affect the performance and production efficiency of the resulting heat-sensitive recording materials.
As described above, since the simultaneous multilayer application by curtain coating method makes it possible to simultaneously apply multiple layers, it is possible to greatly improve productivity. However, after the multilayered coating composition film, is laminated, it is necessary to apply the film onto the support and dry and fix the lamination. If the lamination structure is distorted during lamination or drying and thereby the multiple layers are intermingled, the functions of the respective layers cannot be fully exhibited, thereby impairing the duality as a heat-sensitive recording material. For example, if the layers are intermingled during simultaneous applications, of the heat-sensitive recording layer and the protective layer, it results in uneven film thickness of the protective layer, which decreases chemical resistance to plasticizers or alcohol, or results in insufficient prevention of sticking.
In the field of photosensitive materials or the like in which the simultaneous multilayer application by the curtain coating method has been adopted, the coating composition usually contains gelatin as a binder. Therefore, by cooling a formed layer immediately after the coating liquid thereof is applied onto a support, the gelatin in the coating liquid turns into a gel, allowing the coating liquid to be immobilized. Thereby, the intermingling of the layers is prevented.
In a heat-sensitive recording material, however, the addition of a gelatin in an amount sufficient to immobilize a coating liquid by cooling significantly deteriorates various properties, such as color developing properties and image stability. Methods for immobilizing a coating liquid using a gelatin replacement without sacrificing the foregoing features have not yet been established; therefore, the intermingling of layers cannot be prevented in the same manner as for photosensitive materials.
Several methods have been disclosed as techniques of preventing intermingling of multiple layers of information recording materials such as heat-sensitive recording materials or the inkjet recording materials, thereby desirably obtaining separate layers. For example, Patent Document 3 discloses a method in which, the coating compositions in two adjacent layers become highly viscous with time when they are in contact or mixed. Patent Document 4 discloses a method in which the viscosity of the coating composition that forms multiple layers of the coating composition film is not less than 100 mPa·s, and the surface tension of the coating composition of the undermost layer of the multilayered coating composition film is 18 to 45 mN/m. Further, Patent Document 5 discloses a method in which multiple curtain-coated coating films are dried at a support-to-horizontal surface angle of 45 degrees or less. Patent Document 6 discloses a method in which multiple curtain-coated films are dried within two minutes after the application.
However, in the method disclosed in Patent Document 5, in which the coating compositions in two adjacent layers become highly viscous, the reaction between the two layers generates agglomerates, thereby causing defects in coating. Further, the gelation of a coating liquid may render a long-time continuous operation difficult. Further, in the method disclosed in Patent Document 6, barrier properties to plasticizers or alcohol, and sticking resistance were insufficient. Moreover, the methods disclosed in Patent Documents 5 and 6 suffer from considerable equipment limitations and insufficient quality.
On the other hand, there is disclosed a method of forming an inorganic layer-like compound as one of the multiple layers so as to improve resistance to plasticizers, light stability, and image quality, or to obtain a recording material with a high degree of luster. For example, Patent Document 7 discloses a method of adding mica to the undercoat layer, Patent Documents 8 and 9 disclose a method of adding mica to the middle coating layer, and Patent Document 10 discloses a method of adding mica to one of the layers of the heat-sensitive recording material. Further, Patent Document 11 discloses a method of incorporating kaolin having an aspect ratio of not less than 30 in at least one layer other than the layer between the support and the heat-sensitive recording layer, and Patent Document 12 discloses a method of further providing a protective layer containing kaolin having an aspect ratio of not less than 20 by curtain coating.
However, the method disclosed in Patent Document 7 has a problem in that the increase in viscosity of the undercoat layer decreases its adherence with the support. The methods disclosed in Patent Documents 8, 9, and 10 are effective to some extent. However, depending on the coating method, particularly blade coating, etc., in which a larger shear is created, the orientation of inorganic layered compound particles may be disrupted, resulting in decreased effects of the particles, and the surface strength may be lowered, causing inferior printability, and the like. Furthermore, the methods of Patent Document 11 or 12 using a layer containing kaolin also have several defects including a decrease in printing strength.
Further, Patent Document 13 discloses a method for forming a heat-sensitive recording film material wherein the multilayer simultaneous coating is performed by discharging at least two kinds of coating compositions from separate slits to form a multilayer lamination and making the lamination to free fall on a continuously running web, and wherein the coating liquid which constitutes layers other than a topmost coating layer is formed of a dispersion containing inorganic particles. Furthermore, Patent Document 14 discloses a heat-sensitive recording material comprising a heat-sensitive color-developing layer, a first protective layer, and a second protective layer, which are formed simultaneously by a curtain coating method, wherein the second protective layer contains a specific polyvinyl alcohol and a specific copolymer resin.
Further, there is also disclosed a method of forming a heat-sensitive recording layer using an ethylene-acrylic acid copolymer salt so as to improve resistance to plasticizers, water resistance, heat resistance, and resistance to background fogging. For example, Patent Document 15 discloses a method of providing on a paper support a heat-sensitive recording layer containing, as an adhesive, a water-soluble polymer compound that forms a water resistance film by heating, Patent Document 16 discloses incorporating 1-[α-methyl-α-(4′-hydroxyphenyl)ethyl]-4-(α′,α′-bis(4″-hydroxyphenyl)ethyl]benzene and at least one kind of a salt of an olefin/acrylic acid copolymer as an adhesive in a thermal recording layer, and providing a protective layer on the thermal recording layer, and Patent Document 17 discloses a method of incorporating 1,2-bis(phenoxymethyl)benzene and ethylene-acrylic acid copolymer in the heat-sensitive color developing layer. However, these methods have not succeeded in producing desirable heat-sensitive recording materials, particularly in producing heat-sensitive recording materials with superior resistance to plasticizers. Therefore, there as been a demand for further modification of these technologies.    Patent Document 1: Japanese Examined Patent Publication No. S49-24133    Patent Document 2: Japanese Unexamined Patent Publication No. S57-39985    Patent Document 3: Pamphlet of WO 01/076884    Patent Document 4: Japanese Unexamined Patent Publication No. 2001-18526    Patent Document 5: Japanese Unexamined Patent Publication No. 2001-138632    Patent Document 6: Japanese Unexamined Patent Publication No, 2001-113226    Patent Document 7: Japanese Unexamined Patent Publication No, H11-5366    Patent Document 8: Japanese Unexamined Patent Publication No. H10-193789    Patent Document 9: Japanese Unexamined Patent Publication No. 2004-216718    Patent Document 10: Japanese Unexamined Patent Publication No 2001-199163    Patent Document 11: Japanese Patent No. 3971453    Patent Document 12: Japanese Unexamined Patent Publication No. 2007-253369    Patent Document 13: Japanese Unexamined Patent Publication No 2008-238160    Patent Document 14: Japanese Unexamined Patent Publication No. 2008-260275    Patent Document 15: Japanese Unexamined Patent Publication No, S53-013929    Patent Document 16: Japanese Unexamined Patent Publication No H02-202481    Patent Document 17: Japanese Unexamined Patent Publication. No 2003-072235